highlighting text rather than painting

When I opened a pdf recently, I was asked whether I wanted to convert pages.  I didn't know what this meant, but did so with two pages.  I then saw that the conversion allowed me to highlight text as I would in any document (in straight lines, following the text), rather than use the painting option which moves all over the ighlight portion all over the page like a paintbrush.  I really liked the feature, but I haven't been able to figure out how to format the rest of the pdf that way.  I would appreciate someone explaining to me how to do this.  It was so convenient, and I haven't been able to figure out how to initiate it on my own.  Thank you.